1. Technical Field
The present disclosure relates to optical assemblies for spectral imaging. More particularly, the present disclosure relates to optical assemblies and methods for improving spectral resolution of a color measurement instrument.
2. Background Art
For many manufacturing processes, color quality control is key. Thus, expensive high precision spectrophotometers have been used to “sample” colors of both manufacturing components and finished products. Preferably, color sampling is conducted “on-line” or “on-site,” i.e., in cooperation with a manufacturing process. The benefit of “on line” color sampling is two fold: (1) “on-line” color sampling advantageously enables a more comprehensive inspection of a product line; and (2) “on-line” color sampling facilitates quicker, more effective corrective action, reducing both wasted time and materials. In many instances, manufacturers also advantageously utilize handheld spectrophotometers, e.g., to facilitate mobile/user-driven color sampling.
Due to the nature of manufacturing, “on-line” and handheld spectrophotometers are often exposed to hostile work environments, e.g., auto body shops, construction sites, etc. Thus, a clear need exists for robust inexpensive “on-line” and handheld color measurement systems that are capable of surviving and functioning in such work environments. More particularly, a need exists for “on-line” and handheld spectrophotometers that provide consistently precise color measurements irrespective of temperature variations, shock/vibrations, exposure to particulate or liquid contaminates, etc. Ideally, the sensor technology employed must be very cost effective—essentially to the point where the instrument is relatively expendable—in order to justify deployment in harsh and destructive environments. The difficulty, however, is achieving the desired robustness and cost while maintaining superior spectral resolution and accuracy.
Existing approaches to low-cost industrial color measurement typically utilize one of two general optical configurations: (1) wide-band (i.e., white light) illumination with narrow-band sensing, and (2) narrow-band (i.e., chromatic) illumination with wide-band sensing. A common implementation of the first approach is exemplified by a pulsed xenon illumination source opposite a plurality of spectrally-filtered photodiodes. A common implementation of the second approach is exemplified by a plurality of sequentially pulsing high brightness narrow-band LEDs opposite a simple wide band sensor. The above approaches advantageously strive to increase precision by minimizing the adverse affects of ambient lighting, i.e., via using high intensity illumination and/or differential color measurement.
In considering the above configurations, it is noted that LEDs are rapidly becoming a preferred means of illumination. LEDs are small, relatively inexpensive, energy efficient, bright and durable. Moreover, LEDs provide promising opportunities for further optimization, particularly in the areas of packaging, spectral coverage, and efficiency. LED costs have decreased dramatically and predictably as automation and economies of scale have been applied in the fabrication process. Furthermore, whereas LEDs were originally unable to generate the entire range of colors in the visual spectrum, recent material discoveries and evolutions in the manufacturing process have closed such gaps in LED spectral coverage.
In particular, LEDs offer two distinct advantages over incandescent illumination sources. First, LEDs are capable of emitting light at specific wavelength bands, whereas incandescent light sources require association with optical filters (optical filters are costly and reduce the overall efficiency of the light source). Secondly, unlike incandescent light sources, LEDs are current-driven devices with near instantaneous response times. Thus, the current used to power an LED may be advantageously modulated, e.g., at extremely high frequencies (approximately 1 MHz). As taught in U.S. Pat. No. 6,888,633, entitled “Color Measurement Instrument with Modulated Illumination,” this capacity for frequency modulation may be exploited, e.g., to enhance both the selectivity and sensitivity of the color measurement instrument.
With regard to selectivity, frequency modulation may be used to advantageously distinguish a given light source from ambient light conditions and/or other light sources. In other words, if an LED is modulated at frequency X, the detector may be configured to respond ONLY to light modulated at frequency X. Thus, a wide-band sensor may be used to isolate and detect light originating from a specific LED and having a relatively narrow spectral output. Wide spectral coverage may be advantageously obtained, e.g., by modulating several LEDs, sequentially or concurrently, and later extracting each individual LED sub-signal from the detected signal based on a corresponding modulation “signature.” Thus, a single wide-band sensor may be used to simultaneously measure a plurality of LED channels.
With regard to sensitivity, the above described frequency modulation limits the signal of interest to one or more sub-signals having predetermined frequency(s). Detection/amplification techniques may take advantage of this property to optimize the signal-to-noise ratio for the extracted sub-signals based on the known frequency component(s) thereof (see, e.g., U.S. Pat. No. 7,092,097, entitled “Color Measurement Instrument,” regarding improving the signal to noise ratio and overall sensitivity of an LED-based color measurement instrument using auto-zeroing at the sensor diode). Thus, whereas conventional measurements of non-modulated light are sensitive to signal processing artifacts, such as voltage offsets, stray currents, thermal drift, and random and spurious forms of electronic noise, frequency modulation enables narrow-band detection/amplification of the corresponding sub-signals which can filter, avoid, or submerge such artifacts. Accordingly, frequency modulation techniques may advantageously improve the stability of color measurements and expand the dynamic range of instrumentation based on such measurements.
Despite efforts to date, however, there remains a need for improved optical assemblies which provide greater spectral resolution in hostile work environments (greater spectral resolution enabling, e.g., detection of metameric and high chroma samples). More particularly, a need exists for improved optical assemblies that maximize the spectral resolution of a color measurement instrument, e.g., a color measurement instrument employing multi-band chromatic (LED-based) illumination. These and other needs are satisfied by optical assemblies of the present disclosure.